The present invention relates generally to an electronic image pickup system, particularly to an image pickup system comprising a zoom lens and an image pickup device such as a CCD, and more particularly to a digital camera capable of obtaining electronic images. The present invention is also directed to a video camera or digital camera, the depth dimension of which is reduced by contriving an optical system portion thereof, e.g., a zoom lens. A part of the zoom lens is designed to operate in a rear-focusing mode.
In recent years, digital cameras (electronic cameras) have received attention as cameras of the next generation, an alternative to silver-salt 35 mm-film (usually called Leica format) cameras. Currently available digital cameras are broken down into some categories in a wide range from the high-end type for commercial use to the portable low-end type.
In view of the category of the portable low-end type in particular, the primary object of the present invention is to provide the technology for implementing video or digital cameras whose depth dimension is reduced while ensuring high image quality.
The gravest bottleneck in making the depth dimension of cameras thin is the thickness of an optical system, especially a zoom lens from the surface located nearest to its object side to an image pickup plane. To make use of a collapsible lens mount that allows the optical system to be taken out of a camera body for phototaking and received therein for carrying now becomes mainstream.
However, the thickness of an optical system received in a collapsible lens mount varies largely with the lens type or filter used. Especially in the case of a so-called + precedent type zoom lens wherein a lens group having positive refracting power is positioned nearest to its object side, the thickness of each lens element and dead space are too large to set such requirements as zoom ratios and F-numbers at high values; in other words, the optical system does not become thin as expected, even upon received in the lens mount (JP-A 11-258507).
A − precedent type zoom lens, especially of two or three-group construction is advantageous in this regard. However, this type zoom lens, too, does not become slim upon received in a collapsible lens mount, even when the lens positioned nearest to the object side is formed of a positive lens (JP-A 11-52246), because the lens groups are composed of an increased number of lens elements, and the thickness of lens elements is large.
Among zoom lenses known so far in the art, those set forth typically in JP-A's 11-194274, 11-287953 and 2000-9997 are suitable for use with electronic image pickup systems with improved image-formation capabilities including zoom ratios, field angles and F-numbers, and may possibly be reduced in thickness upon received in collapsible lens mounts.
To make the first lens group thin, it is preferable to make the entrance pupil position shallow; however, the magnification of the second lens group must be increased to this end. For this reason, some considerable load is applied on the second lens group. Thus, it is not only difficult to make the second lens group itself thin but it is also difficult to make correction for aberrations. In addition, the influence of production errors grows. Thickness and size reductions may be achieved by making the size of an image pickup device small. To ensure the same number of pixels, however, the pixel pitch must be diminished and insufficient sensitivity must be covered by the optical system. The same goes true for the influence of diffraction. To obtain a camera body whose depth dimension is reduced, a rear focusing mode wherein the rear lens group is moved for focusing is effective in view of the layout of a driving system. It is then required to single out an optical system less susceptible to aberration fluctuations upon rear focusing.
Referring here to a conventional image pickup system using a zoom lens and an electronic image pickup device, a so-called variable stop with variable aperture diameters has been used primarily for adjusting the quantity of light passing through the zoom lens.
With a view to image quality improvements, on the other hand, image pickup devices of today are required to have ever higher pixel densities. The more the pixels of an image pickup device, the hither the optical performance demanded for an optical system becomes.
A problem with use of a conventional variable stop is, however, that when it is intended to decrease the diameter of the stop thereby adjusting the quantity of light, resolution drops under the influence of diffraction. In other words, it is still difficult to reconcile light quantity adjustment with high image quality. Even when it is intended to shorten the overall length of the zoom lens, the thickness of mechanical construction for the variable stop often puts limitation on the reduction in the whole length of the zoom lens.